Material failure

I have been using a tensiometer to help me try and determine what the material is. one of the materials has been heat treated and and instead of a nice necking formation in the middle of the "dumbell sample" it has fractured perpendicular to the sample right next to the chucks of the tensiometer, rather then the thinner section. what is the reasoning behind this?

presence of porosity at the neck section may be reason.
Improper machining at the neck region leads to improper radius which might cause stress concentration at that region causing failure much before.